Cross-Disciplinary Computational Modeling Throughout the Heliosphere II Posters
Presiding: M Linton, Naval Research Laboratory; M Opher, Jet Propulsion Laboratory, California Institute of Technology
SH13B-01 1330h
Modeling of the Solar Wind with a Tetrahedral MHD Algorithm
MH4D (Magnetohydrodynamics on a TETRAhedral Domain) is a numerical algorithm to solve the resistive and viscous MHD equations on an unstructured mesh of tetrahedra. It is device independent and runs on desktop computers as well as massively-parallel systems. The formulation on an unstructured grid allows the computational domain to be of arbitrary shape and the resolution to be increased in the regions of physical interest. Consequently, a wide range of spatial scales can be studied at the same time, for example active regions can be embedded in the large scale corona. A variational formulation of the differential operators ensures accuracy and the preservation of the analytical properties of the operators (∇ · B=0), and self-adjointness of the resistive and viscous operators. The combined semi-implicit treatment of the waves and implicit formulation of the diffusive operators can accommodate the wide range of time scales present in the solar corona. The capability of mesh refinement and coarsening is also included. We will present a solution of the supersonic solar wind obtained with our model.
SH13B-02 1330h
Modeling Wave Activity in Coronal Active Regions
Wave activity in coronal active regions has been detected recently in EUV and as Doppler velocity oscillations by SOHO, and in EUV by TRACE. The properties of the waves are determined by the excitation mechanism, and by the local physical conditions, such as magnetic field strength and geometry, temperature, and density. It has been demonstrated that the phase speeds of the various wave modes can be determined from observations. Using 3D MHD model I investigate the generation, propagation, and damping of MHD waves in active regions, with the goal of developing a diagnostic tool of active region physical properties. Photospheric magnetograms were used as the boundary conditions for the magnetic field model, and the initial state was constructed using force-free extrapolation, and gravitationally stratified density. Here I expand previous studies, and investigate possible excitation mechanism, as well as the damping of the transverse oscillations due to resistivity, and leakage. I also investigate the generation, propagation, and damping of longitudinal oscillations in the active region loops. In the present study improved boundary and initial conditions are developed, and the effects of chromospheric coupling on the excitation and damping of the waves are investigated.
SH13B-03 1330h
Evolution of CME-driven Shocks in the Lower Corona for the October-November 2003 Events
While it is generally accepted that the largest energetic particle events are created by CME-driven shocks in interplanetary space, the relative importance of CME-driven shocks versus flare-related processes in creating energetic particles low in the corona is not understood and is an area of active research. We analyzed the formation of CME driven shocks in the lower corona for the Halloween Space Storms that occurred in late October and early November 2003. We used the Space Weather Modeling Framework (SWMF) developed at the University of Michigan to create a realistic corona. The CME was modeled as an out of equilibrium flux rope lying under a closed field region in the AR 10486. The MHD code was first used to create realistic background corona using observed photospheric fields for boundary conditions for the Carrington rotation 2008. The background corona was validated by comparing results from the model with in situ solar wind observations from ACE/WIND. We discuss the magnetosonic speed profile in the lower corona and the consequences for the CME shock formation. Consequences for the acceleration of particles to GeV/nucleon are discussed. The computational runs were performed at the supercomputer Columbia at NASA/AMES.
SH13B-04 1330h
MHD Simulations of Coronal Plasma Driven by Boundary Flows.
A large portion of theoretical work on describing the large scale plasma evolution in solar and laboratory plasmas relies on Taylor's theory of relaxation which predicts that a plasma configuration will relax to a constant α profile, i.e., a force-free state with a constant in space coefficient of proportionality between the magnetic field and current density J = α B. Observations, however, show that α in the active region is a complex function of space (e.g. Zhang et al., 2001). In particular, the spatial profile of α consists of adjacent patches of opposite sign. We present a numerical MHD model of the dynamics of the active region plasma under photospheric boundary flows which addresses questions associated with the spatial structure of α. The simulations reveal that the evolution proceeds through a formation of adjacent, nearly force-free regions with parallel and anti-parallel magnetic fields and current densities, i.e. with α of opposite signs. We present the details of the model, explain the structure of the obtained solutions and the reason for the formation of such configurations. The boundary conditions are modeled by imposing a velocity profile and deriving consistent conditions on the magnetic field from the MHD equations. In order to address the numerical difficulties associated with the stiffness of the resistive MHD equations we use new exponential propagation iterative (EPI) methods which allow accurate time integration with time steps exceeding the CFL restriction on explicit schemes.